This invention relates to steam-hydrocarbon reformers, and more particularly to the application of a second reforming section in the design of a steam-hydrocarbon reformer.
Steam-hydrocarbon reforming is widely used to make synthesis gas (syngas) for hydrogen plants, methanol plants, ammonia plants and the like. Synthesis gas is defined herein as a gas comprising hydrogen and carbon monoxide. Hydrogen and syngas are typically produced by reforming a steam/hydrocarbon mixture at high temperature in a fired reformer. Such a reformer contains catalyst filled tubes which convey the feed at elevated pressure through the furnace where it is heated and reacts to generate the syngas product. The furnace is heated by combusting fuel with air outside of the reformer tubes at near ambient pressure. High temperatures are required to achieve good conversion. The efficiency is improved by using the waste heat contained in both the flue gas and the product which exit the fired reformer. This waste heat has traditionally been used to preheat the feed and to generate steam for the process and for steam export. Significant quantities of export steam are generated and must be utilized for an efficient process. Although steam is a useful by-product, it my be difficult to find a customer for the steam or significant expense can be incurred to pipe it to the end-user. In addition, because a significant quantity of fuel is burned to generate steam, the fuel usage and carbon dioxide emissions per unit of hydrogen or syngas generated are high.
Modern reformers employ additional methods to use the waste heat from the reformer in order to reduce the fuel usage, the amount of export steam and the quantity of carbon dioxide emitted. In particular, the air and/or fuel may be heated before combustion in the furnace. Also, the steam/hydrocarbon feeds can be heated separately or combined to higher temperatures. These methods reduce the amount of fuel that is required for a given amount of syngas production. However, materials of construction become expensive or unavailable as the temperatures are raised and the temperature to which the feed and fuel can be preheated is limited by the tendency of the hydrocarbons to crack (form solid carbon which fouls and plugs exchangers) at high temperature.
In order to overcome these limitations the industry has developed specialized designs that recover the waste heat by reforming. The most widely practiced of these processes is an adiabatic prereformer in which the feed is first preheated then directed to an adiabatic vessel filled with specialized prereformer catalyst. The gas partially reforms (converts to hydrogen and carbon monoxide) which reduces the temperature due to the endothermic nature of the reaction. This gas is then further heated against the flue gas stream from the fired reformer and then introduced to the tubes in the fired reformer to achieve the desired outlet conditions. Prereforming reduces the size of the primary reformer since part of the duty is accomplished in the adiabatic reactor. It also reduces the amount of fuel, export steam and carbon dioxide emitted since the partial reforming is accomplished with waste heat. A prereformer can be combined with the other methods mentioned above to further improve the efficiency of syngas generation.
The primary disadvantage of the adiabatic prereformer is the specialized catalyst that is required. It has a high nickel content to achieve good activity at relatively low temperatures and tends to be sensitive to steam. The steam sensitivity requires special operating methods during start-up and shut-down to avoid damaging the catalyst. The adiabatic reformer is also limited in the amount of conversion it can achieve: typically 15% for natural gas feedstocks. This is because reforming of light feedstocks is endothermic (the reforming gas cools in an adiabatic reactor) thereby self-limiting the amount of reforming that can take place.
Regardless of what additional equipment is provided to improve the efficiency with which syngas is generated, the fired reformer is a major cost item in the plant. The tubes in the radiant section of a conventional reforming furnace are generally filled with a catalyst such as nickel on an alumina support. Care must be taken to minimize the formation of coke on the catalyst, as well as the introduction of catalyst-poisoning contaminants in the feed stream supplied to the tubes. Coke formation generally occurs at the entry of the hot feedstock into the tubes, before sufficient hydrogen is present in the gas to inhibit coke formation. Different catalyst, such as small diameter catalyst or potassium promoted catalyst, is used at the tube inlet to reduce the potential for coke formation. The catalyst at the tube inlet is also more susceptible to deactivate in the event catalyst poisons are fed into the tubes.
Prereforming reduces the cost of the fired reformer by shifting the duty to the prereforming reactor. However, there are additional benefits. In particular, hydrocarbons higher than methane have a greater tendency to form coke on the catalyst when operated at high temperature. This limits the heat flux that the reformer can be designed for and expected to operate for long periods between catalyst change-out. Prereforming converts the hydrocarbons heavier than methane in the prereformer and retains catalyst poisons which allows the primary reformer to be designed for higher heat flux resulting in a smaller, less expensive reformer.
Various methods have been proposed that include heating of the prereformer with the waste heat from the flue gas. These earlier methods lack the flexibility needed to permit independent optimization of multiple reforming stages.
It would be useful to have an apparatus and process whereby multiple reforming stages may be optimized independently, thereby having higher efficiency, lower capital costs, and reduced maintenance.
Related patents include U.S. Pat. No. 3,094,391 to Mader, U.S. Pat. No. 6,818,028 to Barnett et al., and U.S. Pat. No. 4,959,079 to Grotz et al.